To test chemicals of interest to the NTP for their ability to cause chromosome damage in vitro, as measured by the induction of chromosome aberrations and sister chromatid exchanges, and in vivo, as measured by the induction of chromosome aberrations, sister chromatid exchanges, and micronuclei, in bone marrow cells of mice and rats. The results obtained from these tests will be used to assist in the evaluation of other in vivo toxicity test results, and will be compared with the responses of the same chemicals in other genetic test systems.